Automatic regulation in radio receivers



July 11, 1939. 3 w i I 2,165,596

AUTOIATIC REGULATION IN RADIO RECEIVERS Filed July 22, 1936 2Sheets-Sheet 1 i w I w *5 I u E! S 3!: g: m I Q Q5 ml E' Q1 III .4: I

2 w? a sq I i I E I N g D I Q Q Q g k l l K 5 $1 v E Y E g Q R s070/!51-78/11 m k R Q E Q 9170/1 All 4001/10 SIDNEY Y. WHITE INVENTORATTORNEY July 11, 1939 w n- 2,165,596

AUTOMATIC REGULATION IN RADIO RECEIVERS Filed Jul 22', 19se 2Sheets-Sheet 2 l 000 JFEJKEE SIDNEY Y WHITE, INVENTOR ATTORNEY PatentedJuly 11, 1939 UNITED STATES AUTOMATIC Sidney Y.

REGULATION IN RADIO RECEIVE White, Chicago, 111., assignor to Victor S.Johnson, Chicago, Ill.

Application July 22, 1936, Serial No. 91,945

20 Claims.

This invention relates to improvements in high-frequency signalingsystems, and more particularly to systems employing modulatedhighirequency signals for the transmission of intellirs gence. Morespecifically, this invention relates to improvements in radio broadcastreceivers or other types of radio receivers adapted to respond to aparticular desired signal.

An object of this invention is to provide an improved type of noisesuppression means for radio receivers.

An additional object of the invention is to provide means for preventingappreciable response from a radio receiver in the absence of an input 5signal having a predetermined initial minimum amplitude.

Still a further object of the invention is to provide means formaintaining the output signal of a radio receiver substantially constantas the amplitude of an input signal, the initial amplitude of. whichexceeds a predetermined minimum value, varies over a wide range.

Another object of this invention is to provide a definite and'widelyadjustable threshold value for the input signal to a radio, receiverbelow which no response is obtained, and above which the response ismaintained substantially uniform at a desired level.

Radio receivers which are equipped with means for automaticallycontrolling their ampliflcation inversely in accordance with thestrength of the input signal have the disadvantage that theamplification automatically rises to a maximum degree when the receiveris not tuned to a signal of at least-moderate strength.

, This rise in amplification or sensitivity produces a great increase inthe response of the receiver to signals too weak to be useful, and toextraneous disturbances, so that the listener may hear harsh andunpleasant sounds as the receiver is tuned from one strong signal to thenext. This difficulty is not overcome by retarding the manual volumecontrol of the receiver, since to do so also reduces below audibilitythe response to the signals from which a selectionis to be made.

Various types of noise gates and sensitivity controls have, in the past,been employedto minimize this inherent disadvantage of receivers havingautomatic amplification control means. The first-mentioned devices alldepended upon the amplitude of the input signal for operation, so thatonly relatively strong signals were able to get through the gate andproduce a response. Likewise, however, extraneous disturbances such 5 asstatic and other undesirable electrical noises were able to get throughthe gate and produce a response, provided they were of sufficientstrength. Noise gates had the further disadvan- 'tage of producingserious distortion of an input signal having an amplitude close to thethreshold value for which the gate was initially adjusted. For thesereasons, noise gates are not a satisfactory solution of this problem ina receiver having automatically controlled amplification.

Another attack on this problem was made by employing means for limitingthe maximum sensitivity of the receiver. In this way, the performance ofthe receiver on relatively powerful input signals was not appreciablyaffected, but its response to relatively weak undesired disturbances wassubstantially reduced. Although reducing the maximum sensitivity of thereceiver was partially successful in preventing signals too weak to beuseful and extraneous noises from being objectionable, this method hadthe distinct disadvantage of greatly decreasing the ability of. thereceiver to cope with fading of desired signals. As the signal faded,its amplitude would fall below the threshold value for which thesensitivity control had been adjusted, and nousefulresponse would thenbe obtained. To keep extraneous response at a minimum between thedesired signals, the threshold value had to be set relatively high,. sothat even strong input signals frequently fadedbelow the thresholdvalue. Providing means for regulating the maximum sensitivity of areceiver, therefore, is asatisfac tory method of preventing anyappreciable response only when the input signal initially eX- ceeds apredetermined minimum value and never fades below that value.

The arrangements of the present invention, however, provide an improvedform of noise suppression system which discriminates between in putnoise and input signals and. which is automatically unlocked only by aninput signal having a predetermined initial minimum amplitude,Furthermore, the presence of such an input signal automatically lowersthe threshold value in such a manner that the signal after beingreceived may vary over a wide range, due to fading or other changes intransmission, without falling below the new threshold value ofthereceiver. An additional advantage of the arrangements hereindisclosed is that the adjustable threshold value is definite anddistinct and there is no distortion of a signal having an amplitude onlyslightly greater than the threshold value.

Although the arrangements of the present invention are adapted for usein conjunction with any type of radio receiver, they are particularlyuseful in receivers which are provided with means for automaticallyregulating their amplification, and 'are especially suitable for use inreceivers also employing automatic resonanceadjusting means, such, forexample, as disclosed in my co-pendlng application for Letters Patent,No. 29,244, filed July 1, 1935. When used in conjunction with such asystem, the arrangements herein disclosed: are capable of providing \aradio receiver which responds only to signals of a predetermined initialminimum amplitude, to which the receiver is automatically correctlytuned.

- It is a further object of the present invention, therefore, toprovidea noise suppression system particularly adapted to and having especialad vantages when used in conjunction with systems.

including automaticresonance adjusting means;

' When used in conjunction with such systems, the arrangements of thepresent invention permit adjustment of the minimum signal to which theautomatic resonance adjusting means will respond, and-thus eliminate theunpleasant sounds which might otherwise be produced due to the highamplification existing when the receiver is adjusted to relatively weaksignals.

Additionally, and of even greater importance in conjunction with systemsincorporating automatic resonance adjusting means, when the various.components of the arrangements about to be describedare chosen to havethe proper values,

the automatic resonance adjusting 'means is caused to relinquish itscontrol with respect to any particular signal when the tuningmeans hasbeen advanced a predetermined amount beyond the point corresponding toresonance'with that signal.

will relinquish control when the tuning means. has been advanced areasonable amount beyond the signal. When the receiver is tuned from onereasonably strong signal directly into another reasonably strong signal,this problem is not difllcult of solution; but when tuning proceeds intoa region in which there is no signal or only a very weak one, theautomatic resonance adjusting means does not relinquish its control asit should,

- mployed.

but continues to maintain the receiver tuned to the signal to which itwas previously-adjusted.

It will be understood, therefore, that my present invention contemplatesseveral highly advantageous improvements applicable to automaticresonance adjusting systems, but also having utility of a high order inreceivers in which no automatic resonance adjusting means are The aboveand other objects of my invention will more clearly appear by referenceto the accompanying drawings illustrating a preferred embodimentthereof, in which: Fig. lis a schematic diagram of one form of theinv'entionfincorporated in a portion of a radio receiver:

Fig. 2 is a schematic diagram of a superheterodyne radio receiverincorporating both the arrangements of the present invention and theautomatic resonance adjusting means disclosed in my co-pending'application for Letters Patent, No. 29,244; 1

v I is grounded.

3 is a graph showing the input vs. output "characteristic of the radioreceiver. a portion of tuned circuit 2 and its plate or output circuitincluding tuned circuit 3. Cathode 4 of vacuum tube I is grounded.By-pass capacitors I and 5a ofier a low-impedance path forhigh-frequency currents in the input and screen-grid circuits,respectively, of vacuum tube I.

Tuned circuit 3 is coupled to tuned circuit 6, which is included in thegrid or input circuit of vacuum tube I. The input circuit of vacuum tubeI also includes, filter resistor 8 and volume-control potentiometer 9.Cathode ID of vacuum tube Capacitors II provide low-impedance paths forthe high-frequency currents in the input circuit of vacuum tube I; Theplate circuit of vacuum tube I includes load resistor I2 and thescreen-grid I3 of vacuum tube I is by-passed to ground by means ofcapacitor ll. Plate II of vacuum tube I may be connected to screen-gridII by means of switch I6. Screengrid 23 of vacuum tube I is connected toplate I5 of vacuum tube I.

Vacuum tube I I includes two diode-rectiflers and a triodeaudio-frequency amplifier. The grid I8 01 the triode portion isconnected through capacitor I9 to the movable arm of volume-controlpotentiometer 9. The plate circuit of the triode portion of vacuum tubeI I includes load resistor and filter resistor 2|. Cathode 22 of vacuumtube I1 is grounded.

Resistors 24, 25 and 26 are connected in series between a source ofnegative voltage E and plate I5 of vacuum tube I. Grid-leak 21 isconnected between the junction of resistors 24 and 25 and grid I 8 ofvacuum tube I'I. Capacitor 28 bypasses the junction of resistors 2! and25 to ground. The junction of resistors 25 and 28 is connected to diodeanode 29 of vacuum tube II. Resistors 30 and 3| are connected in seriesbetween source of negative voltage E and ground. Resistor 30 is shuntedby potentiometer I2 and resistor 33 in series.

Control-grid 34 of automatic amplification con trol amplifying vacuumtube II is connected to control-grid 36 of vacuum tube I.

vacuum tube 3! includes the primary winding of transformer 38. Oneterminal of the secondary winding of'transformer 3| is connected todiode anode 39 of vacuum tube II. The other terminal is connectedthrough resistors ill and 4| in series to the movable arm ofpotentiometer 32. The

low-potential terminal of tuned circuit 2 is connected through resistor42 to the junction of recapacitor 43. s

In operation, let it first be assumed that no signal voltage is presentacross tuned circuit 2. The control-grid l6 oi vacuum tube I has sub-Cathode 31 .of

vacuum'tube 35 is grounded. The plate circuit of 66 v stantially thesame potential as does cathode III of vacuum tube I, and therefore this,tube draws a relatively high,plate current. The high plate current oivacuum tube I flowing through re- 3 plate II of tube I.

sistor I2 produces a large voltage drop, so that vacuum tube I may havea positive potential of only about 25 volts. Since screen grid 22 ofvacuum tube I is connected to plate ii of vacuum tube 1, theamplification of intermediate-frequency amplifying vacuum tube I isterially reduced due to the very low screen-grid voltage. I tube I isdetermined by the-adjustment of potentiometer 32, which alters thedirect-current potential applied to control-grid 3B- of vacuum Thevalues of resistors 24, 25 and 26 are so chosen that, inthe no-signalcondition, a negative bias potential suflicient to block the tube isapplied to grid II of the triode portion of vac-- uum tube I1. Withcurrent of the tricde portion of vacuum tube I1 is substantially zeroand' the tube is therefore incapableof amplification.

.Now let it be assumed that a relatively strong signal voltage appearsacross tuned circuit 2. Even though theampliflcation ofintermediatefrequency amplifying vacuum tube I is low, sufllcient signalvoltage appears across tuned circuit 6 to develop a potential dropacross.resistor I and potentiometer 9 in series because otherwiseunbiased control-grid 4.5 of vacuum tube 1, with cathode It), .acts as adiode rectifier. This voltage drop causes control-grid 46 of vacuum tubeI to become negative with respect to cathode III, producing asubstantial decrease in the plate current of vacuum tube I. This in turnreduces the potential drop across resistor I2 and increases the voltageapplied to screen-grid 23 of intermediate-frequency amplifying vacuumtube I, with a resultant increase in the amplification of vacuum tube I.

This further increases the signal across tuned circuit 8, which againdecreases the plate current of vacuum tube I, resulting in a cumulativeunstable condition which continues until full am-' pliiication has beenrestored to vacuum tube I,

and the threshold has in eflect disappeared. The

rate at which the amplification is restored depends'upon the value ofcapacitor a. a

To re-establlsh this threshold, it is necessary that the input signalvoltage decrease, either by detuning or due to severe fading, to a valuewhere the signal, despite full gain in vacuum tube 'I,

is too weak to maintain a substantial negative bias on control-grid 46of vacuum tube I. Under these conditions, the plate current of vacuumtube 1 increases, with consequent greater voltage drop in resistor l2,thus decreasing the voltage on the screen-grid of vacuum tube I, furtherdecreasing the signal, and another cumulative unbalance occurs, endingin the original threshold being re-established.

The increase in the potential of plate I5 of vacuum tube 1 causes diodeanode 29 to become positive with respectto cathode 22 of vacuum tube I1.Since the internal resistance of "the diode rectifier is now smallcompared'with the values of resistors 24, 25 and 26, the junction ofresistors 25 and 26 is effectually grounded. The

values of resistors 24 and 25. are so chosen that the proper negativebias voltage is applied to grid I8 of the triode portion of vacuum tubeI1 for normal operation as an audio-frequency amplifier. The rate atwhich vacuum tube I1 is restored to normal operation is determined bythe value of capacitor 28.

The audio-frequency component of the signal appearsacross'volume-control potentiometer 9,

The actual amplification of vacuum this bias voltage, the plate grid 45of vacuum tube 1 operates simultaneously and as the control-grid as theanode of a 'diode v of atetrode, the tetrode functioning to amplify the'direct current potential variations which appear across resistor 8 andpotentiometer in series.

By making the time required for unblochns the audio-frequency amplifyingvacuum tube' I'l considerably greaterthan that required to restore theamplification of intermediate-frequency amplifying vacuum tube I,the'system is made unresponsive to bursts of static'and other electricaldisturbances of shortduration regardless of their amplitude. Acontinuous signal of predetermined initial minimum amplitude, however,first restores the amplification of intermediate- 'frequency amplifyingvacuum tube I'and then unblocks the audio-fretfuency amplifying portionof vacuum tube II. This definite order oi opera-' tion has theadditional and important advantage of preventing response in the to ofan output signal across resistor until the trength of the input signalis well above the threshold value, thuspreventing any distortion whichmight occur at or near the threshold value from being present in theoutput of the system.

Ths threshold distortion is of two kinds. If

change its plate current. Since vacuum tube I1 is blocked until adefinite amount of change of plate current of vacuum tube 1 occurs, thereceiver produces no sound while the signal is breaking down thisthreshold.

The other kind of threshold distortion occurs I during the period inwhich the grid bias ofvacuum tube I1 is decreasing from the blocked tothe operating condition. cumulative snap interaction of vacuumjtubes Iand 'l', the grid-bias'voltage' on vacuum tube I-l is changed so rapidlypassed through and consequently is not noticeable.

However, due to the that this distortion is in a small fraction of asecond,.

Since the amplification of intermediate-frequency amplifying vacuum tubeI is substantially increased by the presence of an input signal havingan amplitude above a' predetermined initial minimum value, ered as aresult. Thus. there'is a'reserve of ampliflcation to aid incounteracting fading'or other changes in the strength of the inputsignal. This automatic lowering of; the threshold value is an importantfeature of the present invention.

Theivoltage across tuned circuit 2 is also applied to control-grid 34 ofautomatic-ampliflcation control amplifying vacuum tube .35. After beingamplified by vacuum tube 35, the signal the threshold value is low.-

voltage is applied to diode anode 39 of.vacuum tube II by means ofcoupling transformer 38. Diode anode 39 is initially negative withrespect to ground by an amount dependent upon the setting ofpotentiometer 32. when the signal voltage is sufficiently high toovercome this initial negative bias, a direct-current voltage isdeveloped across diode load resistors 40 and II in series. This voltageis utilized to provide automatic amplification control of the vacuumtubes preceding vacuum tube l in the receiver, series resistor 44 andshunt capacitor 45 preventing l and 43a. Thus vacuum tube I is regulatedto a lesser extent than are the preceding vacuum fluctuations due tomodulation of the signal voltage from reaching the controlled vacuumtubes, the control-grids of which maybe operatively connected to thejunction of resistor and capacitor 45 at AVC.

The control voltage which is developed across resistor ll is applied tothe control-grid of intermediate-frequency amplifying vacuum tube I, thefluctuations due to modulation being removed by series resistor 42 andshunt capacitors tubes by the automatic amplification control means, butat the same time contributes toward the over-all effectiveness of thiscontrol means.-

In no event, however, is the amplification of this vacuum tube'reducedby the automatic amplification control means to an extent sumcient todeprive vacuum tube 1 of the signal voltage necessary to maintain itsplate current at a low value. The fact that the present inventiondiscloses an automatic noise suppression system which does not interferewit or appreciably limit the range of operation 'of t e automaticamplification control means is another of its important features.

The setting of potentiometer 32 determines the initial negative biasvoltage which is applied to the control-grids of the preceding vacuumtubes as well as that applied to control-grid 36 ofintermediate-frequency amplifying vacuum tube I. This potentiometerfunctions therefore, both as means for adjusting the maximum sensitivityof the receiver and as an adjustment ofthe SI" initial threshold valueof input signal to which the receiver is'responsive. For example, movingthe arm of potentiometer 32 toward the terminal which is connected toresistor 33 decreases the maximum sensitivity of the receiver andsimultaneously raisesthe threshold value, adapting the receiver toreceive strong signals with a minimum of extraneous noise betweensignals. Moving the arm of potentiometer 82 toward the Junction ofresistors SI] and II, however, increases the maximum sensitivity of thereceiver and at the same time decreases the threshold value, thusadapting the receiver for the receptionof. rela-' tively weak signals.An important feature-of the present invention is that only a singlecontrol is required to adjust the system for any particular condition ofoperation, thus greatly simplifying the construction and operation ofthe complete receiver. The entire noise suppression system is renderedinoperative merely by throwing switch It to the lower position, and thisswitch may readily be arranged so that it is operated by a slightmovement of the arm of potentiometer I! at the high-sensitivity end. The

switch connects plate I! of vacuum tube 1 and screen-grid 23 of vacuumtube l to the screensupply point B, restoring full amplification to.vacuum tube I and ensuring correct bias for vacuum tube l1.

It will be understood that the noise suppression system Just describeddiffers in fundamental particulars from the various noise gates and"sensitivity contro of the prior art, in that it comprises asequentialseries of automatic operations, this series being inherentlynbn-reversible. It will also be understood thatthis noise suppressiorisystem, in and of itself, has highly advantageous application not onlyin radio receivers but equally as well in any device or apparatus'inwhich it is desired to establish oper- --at ive conditions in responseto initial conditions,

without regard to subsequent fluctuations. Also, in radio receivers, itis possible by suitable choice of constants to cause the neisesuppression system-to operate in response to relatively small changes insignal level at the demodulator, such as might result if the receiver istuned only slightly away from resonance with a desired signal. Such anarrangement prevents any response from theloud speaker. under conditionswhich might otherwise produce seriousdistortion,

and operates to give a\res ult similar to that secured by automaticresonance adjustment.

Fig. 2 is the schematic diagram of a multiband superheterodyne radioreceiver which incorporates not only the improved noise suppressionsystem which is shown in Fig. 1, but also the automatic resonance.adjusting means which is disclosed in my copendi'ng application forLetters Patent, No. 29,244. This receiver. employs thirteen vacuum tubesarranged as follows: a

radio-frequency amplifying vacuum tube 46; a

modulator vacuum tube- 41; a decal oscillator vacuum tube 48; a firstintermediate-frequency amplifying'vacuum tube I; a combined demodulator,second intermediate-frequency amplifying and direct-current amplifyingvacuum tube 1; a combined first audio-frequency amplifying automaticamplification control rectifying and bias voltage determining vacuumtube ll; a second audio-frequency amplifiying'vacuum tube 49;. apush-pull audio-frequency output stage consisting of vacuum tubes 50 and5!; a power.

rectifier vacuum tube 52; an automatic ampli-- fication controlamplifying vacuum tube '35; a

director vacuum tube 53; and a corrector vacuum-tube 54'. With theexception of the novel features herein disclosed and those disclosed inmy co-pending application for Letters Patent, No. 29,244, the receiveris of conventional design 5 and operates in the usual manner,

As between Figs. 1 and 2, corresponding parts are designated by the samenumeral. It is unnecessary, therefore, to repeat a detailed descriptionof the novel noise suppression system-herein disclosed as to itsoperation in the receiver of Fig. 2, except insofar as the resultsobtained differ from those stated in the discussion of Fig. 1. Inaddition to functioning as a rectifier and as a direct-currentamplifier, vacuum tube 1 serves as an intermediate-frequency amplifier.The plate circuit of vacuum tube I includes series inductor 55aand-coils 55 which are coupled with coils 56a. and SBb-associated withvacuum tube 53 to form the director portion of the automatic frequencyadjusting system. The director is arranged to supply a direct-currentvoltage to control-grid 51 of vacuum tube 54, which is arranged tofunction as the corrector portion of the automatic frequency adjustingmeans. The

input capacitance of vacuum tube 54, in series with capacitor 58, is inshunt with the plate load of oscillator vacuum tube 48.

In the presence of no input signal or of a signal having a value lowerthan the predetermined threshold value for which the receivei may besolely by the potential at tap A on the powersupply divider network. Thevalue of the voltage at tap A is so chosen that the input capacitance ofvacuum tube 54 is approximately at its midpoint. g I

when the initial amplitude of the input signal exceeds the thresholdvalue, the amplification of vacuum tube I is greatly increased by theaction of the automatic noise suppression system, with the result that astrong signal current flows through coils II in the plate circuit ofvacuum tube I. Since coils 56a and 58b are tuned respectively to afrequency slightly lower than the intermediate frequency of the receiverand to a frequency slightly higher than the intermediate frequency, thepolarity of the direct-current voltage which is supplied by the directorto the corrector depends upon whether the frequency of the signalcurrent in coils is higher or lower than the nominal intermediatefrequency of the receiver. If the signal frequency is exactlycorrector-depend respectively upon the amplitude and the polarity of thevoltage supplied by the .voltage from reaching vacuum tube. 53.

director.

The high-frequency amplification of the re-.

ceiver is regulated automatically by a system which includes amplifyingvacuum tube 35 and diode anode I) of vacuum tube II. A direct-currentcontrol voltage is develo'bed across diode load resistors I. and ll inseries, when the signal voltage across the secondary winding of trans-'-former 3| exceeds a minimum value determined by the setting ofpotentiometer 32. The full control voltage is applied to thecontrol-grids of vacuum tubes 46 and 41, and a portion of the controlvoltage is applied to the control-grid of vacuum tube i. The performanceof the delayed, amplified automatic amplification control is such thatthe signal voltage across tuned circuit t remains substantially constantas the input signal voltage varies over a wide range.

When switch it is thrown'to the lower position, not only is theautomatic noise suppression system rendered inoperative as explainedabove in connection with Fig. l, but also the automatic frequencyadjusting means is no longer in operation, since anegative-voltage isapplied to screengrid l3 of vacuum tube I, rendering it incapable ofamplification and thus preventing any signal With switch II in the lawerposition, the receiver functions just as does anystandardsuperheterodyne, but withthe switch in theupper position, thenormal operation of the receiver is supplemented by the automaticfrequency control system and by the automatic noise suppression systemwhich are associated with it.

As has already been pointed out, a major difficulty in the design ofautomatic resonance adlusting systems lies in so arranging the systemthat it will relinquish a station once it has been tuned in. Theintermediate-frequency amplifier inherently tends to stay locked on thestation as the tuning dial of the receiver is turned, until the limit ofoscillator frequency control has been reached, when the oscillatorfrequency is again controlled by the dial movement. If this dialmovement be further continued, the oscillator is forced so far out ofcorrect relation with the signal that the selective frequency cut-off ofthe intermediate-frequency amplifier is approached,

I a limiter tube.

'cel quite completely.

re-ali'gning the oscillator with the dial. s mg. During this process,however, the signal h departed widely from true resonance with theintermediate-frequency amplifier, and serious. distor-' 1. The amount ofcontrol on the oscillator must be limited and must be uniform throughoutthe band. This is necessary in order that a reasonable amount ofmovement of the dial will exhaust the control possibilities of thesystem, and restore control to the dial.

2. The director voltage must belimited, and must be relativelyindependent of signal strength; This is accomplished in a two-foldmanner, first by the automatic amplification control and second byoperating the tube driving the director as 3. The voltage delivered bythe director ordinarily contains transients of considerable mag- Acrosseach diode load resistor there will also appear audio-frequency voltagesand sy-llabic modulation transients. When the intermediatefrequencyamplifier is in accurate relation with the signal, these audio-frequencyand transient voltages are in phase opposition and are substantiallyequal to each other and therefore can- I However, as soon as theoscillator departs from correct relation with the signal, theaudio-frequency and transient volt- I ages produced by one diode will besubstantially greater than those produc'ed'by the other and' completecancellation will no longer occur. There will therefore be superimposedon the steady di-- rector voltage a series of transients of substantialamplitude. These transients I do not filter out. When control tube 54 isat the limit of control due to theisteady direct voltage applied to itsgrid 51, any substantial relaxing of the control voltage will allow theoscillator to snap back into alignment with the dial setting, which maybe for instance 15 kilocycles to one side of the setting exactlycorresponding to the frequency of the signal. Thetransients 'of randomamplitude increase as the oscillator is mistuned, and finally reach anamplitude sufficient to partly cancel the steady direct-currentcomponent of the director output voltage, If desired, these transientsmay be separately amplified and re-introduced to inthus loses control,the oscillator frequency changes to correspond withthe dial setting, and

,the signal has thus been relinquished.

The effect of the audio-frequency transients in causing the corrector torelinquish control .of the oscillator when'the receiver is detuned, ismade more pronounced by resistor 59a,- shunt'ed by capacitor 59, andresistor 59!; connected in series from the director unit to ground. Itwill be noted that resistorsisa and 591 form a potena1- dividing system,the corrector to their junction. The values of these two resistors arepreferably so chosen that about one-' fourth of the direct-currentvoltage delivered by the director is applied to the corrector. As thereceiver is mistuned, the direct-current voltage across resistors 59aand 59b rises, and then, say when the tuning means is about 12kilocycles from resonance, begins to fall. During the-process of tuningawayfrom the signal, the audiofrequency transients have been graduallyincreasing. By virtue of capacitor 58, which is preferably of a fairlylarge value, the audio-frequency transients are 'by-passed acrossresistor 59a and generate varying voltages only across resistor Theeffect of the combination of resistors 58a and 59b and capacitor 59connected as shown in Fig. 2, therefore, is to increase theeffectiveness of the transients in varying the voltage applied to grid51 of corrector vacuum tube 54 in such a manner that as soon asthedirectcurrent voltage delivered by the director begins to fall, acondition of cumulative instability occurs-and corrector vacuum'tube 54relinquishes control of the frequency of oscillator vacuum tube I I.

Additionally, the arrangement of resistors 59a and 59b and capacitor 59is'such that-when the voltage delivered by the director fails, capacitor59 discharges not only through resistor 59a which is of a very largevalue-"but also through resistor 59b, whichis of much smaller value, and

the current through resistor 59b is in a direction opposite to thatwhich flows when the direc-.

tor is-delivering appreciable voltage. In the interval during whichcapacitor 59 is discharging, therefore, there is an actual reversal ofpotential at the junction of resistors 59a and 59b withrespectto'ground. This reversed voltage is applied to grid 51 of correctorvacuum tube 54 and produces a change in the frequency of the oscillatorsuch as to immediately cause it to' bring into approximate resonancewith the intermediate-frequency amplifier the signal in the adja centchannel, if such signal is present.

An analysis of the behavior of r tors 59a and 59b and capacitorindicates that the same effect which produces actual reversal ofpotential .on grid 51 of vacuum tube 54 when the voltage from thedirector fails or falls to zero, operates to enhance the condition ofinstability'in tlie region between one signal and another in such a wayas tocause an instantaneous .release of one signal and capture of thenext and to narrow theregionin which this exchange occurs so that theoperation of the automatic frequency adjusting means :becomes entirelydefinite and all-tendency to continue to maintain resonance with theundesired signal disappears.

By suitable choice of values for resistors 59a and 59b and capacitor 59'it is possible 'to so emphasize the effects above described that raleaseoccurs just as close to the position of exact resonance as does theinitial operation of the automatic frequencyiadjusting means asthereceiver is tunedinto the signal. I havefound,

however, that it is preferable to so choose these values that release.does not occur quite so close to the position of resonance as does.initlal capture (if the signal.

If the receiver is tuned into from 1 aboveand also from below thefrequency of resocurs are noted.'a "capture band wid nance, and thefrequenciesat which capture oc- M be determined. Similarly, if'thereceiver is tuned away from the signal, iirstby increasing the resonantfrequency of the receiver and then by decreasing it. andthe frequenciesat'which release occurs are noted, a release band width" may bedetermined. I have found that the best performance is obtained whenthe"capture band width" is about 85% of the "release band width". Itwill be understood, however, that by suitable choice of constants, atthe discretion of the designer, various relations of these two bandwidths can be achieved, including the condition in which they aresubstantially equal.

In thetabulation of constants which I give below, asan illustrativeembodiment of my in vention; the values of resistors 59a and 59b andcapacitor 59 are such as to give approximately 'the 85% relation of thecapture and release band widths described above as preferable.

It will be understood that when in the specifl-. cation and claims Irefer to capture of a signal, V

I use this phrase to identify the operation by which the automaticfrequency adjusting means acts to change the oscillator to a frequencyap-.

proximately correct toproduce thedesired intermediate frequencyfor thatparticular signal. Similarly. I refer to release of asignal when I wishto describe the operation by which the automatic frequency adjustingmeans ceases to maintain the frequency of the oscillator at approxi- Imately the correct value to produce the desired intermediate frequencyfor that particular signal.

It will be understood that by suitable choice of constants, the noisesuppression syste'rn' first described maybe made to operate at.positions closer to the position of exact resonance than they release.system just. described. 'With the 'constants given below, the releasesystem will operate closer' to the position of resonancethan' will thenoise suppressiofi system, its mode of operation being through theautomatic resonance adlusting system. With other constants, however, thenoise suppression system will operate closer to the point of resonance,its-mode of operation being to decrease'the sensitivity of the receiverand at the same time .render the audio-frequency output systeminoperative When-the noise sup pression system operates in the mannerjust. de.-'

scribed, the automatic'resonanceadjusting sysimmediately'relinquishesits control of the frequency of'the oscillator; due to failure of the 1signal voltage at the'demodulator grid.

'Rhe value ofcapacitor is of considerable comeoperative and a sound.will be heard. If-

capacltor 28. is made very large, on the other hand, the grid biasvoltage on vacuum tube. I! will change so gradually that when a desired'signal is first heard it will be relatively faint, and it will increasein loudness over an appreciable interval before it reaches its finallevel. Under these circumstances, rapid operation of the tuning meansmaycause the receiver to pass completely through a signal of sufficientinitial intensity to otherwise-be heard, without pro- Under these.

ducing an audible sound from the loudspeaker.

Thus rapid operation of the tuning means will produce a condition of"silent tuning" in which,

unwanted signals may passed over without being heard, but as soon as thevicinity of a desired station is reached, a slower operation or thetuning meanswill ensure hearing it.

that this is not to'be taken as in any way limiting .my invention, sinceother types and values of components may be employed in a circuitarrangement identical with that shown in Fig, 3,

or in modifications thereof.

' I Reference numeral 'iypeor value Vacuum tube 1, 7, 35, 46 0117. Vtube 17 1 BQI.

83-V.. V tube 53 H6. Resistors 8, 2i, Z4 .060 negohm P 9-- .25 megohm.Ca f 11- 200 micromicroiaradsa -R" .023

Capacitor i4 .025 microfai-ad. Capadtors 10,28, man. .05 microisrad.Resismrm 1 .100 Resistor 25. .015 mogobm. Real-swim .lccmegobm.

.600 .250 n 'iegohm 0.1 mlcroiarad. 2.0

volts.

Fig. .31shows graphically the performance of a radio receiver, a portionof the circuit of which '40 is shown in Fig. 1. .Cu'rve A represents therelation between input and output voltages which is 45 the outputvoltage begins to. increase gradually as the input voltage increases andtends to level on at the high-input end as the automatic ampliflcafloncontrol"system becomes me sly et- Its advantages may be realized inreceivers in which {the high-frequency amplification takes fective.Small input voltages produce an appre- 60 ciable response. time B showsthe performance a predetermined threshold value, but when the 1 Thearrangements of h present invention are of the system when the automaticnoise suppression system is in operation. that is, with switch Ii ofFig. 1 open. In this case, no output voltage 4 is obtained untiLthe lnitial input voltage reaches input voltage increases even slightly abovethis value the output voltage rises to a point which imum' input voltagewhich will produce a response. Curve 0 indicates the effect of loweringthe threshold value, and curve D indicates the performance obtained whenthe threshold value is raised, By properly adjusting potentiometer 32 ofFlg.,1, the threshold value may be made just high enough to permitresponse to signals of desired initial amplitude without allowing weakersignals and extraneous disturbances to produce a response.

so described hereinabove.

Referring to Fig. 4, triangles 80 -85 represent initial input signals ofvarious amplitudes. will be understood that the input signals are thusonly symbolically represented in this figure. The signal voltage at theinput to the demodulator is represented by rectangles 66, 61 and B0,corresponding with input. signals 60, 83 and 65, re-.

spectively. Input signals 6!, 62 and 64, being below thethreshold valuefor which the receiver is adjusted, do not produce any response, due tothe action of the noise suppression system. Al though input signals-60,63 and vary considerably in amplitude. the amplitude of thecorresponding output signals 66, '61 and 68, respecequipped with theautomatic noise suppression system ofthe present invention and with theautomatic frequencyvadjusting means disclosed in my co-pendingapplication for United States Letters Patent, No. 29,244, produces aresponse only to input signals exceeding a predetermined initial minimumamplitude and only when. the

.sponds to one relatively strong signal after another with substantialsllence between these signals. Moreover, no signal is'heard unless thereceiver'is automatically in correctadjustment for the reception ofthatsignal.

It will be understood that Figs. 3 and 4 are not intended .to showgraphically the automatic decrease of the threshold value upon thereception of an input signal having a predetermined initial minimumamplitude, which is one of the important features of thepresentinvention and, is

The application of the present invention is not limited to anyparticular type of radio receiver.

place at the frequency of the incoming signal as well as in receivers ofthe superheterodyne type, in. which the greater part .of theamplification before final demodulation is accomplished at a relativelylow constant intermediate frequency.

not limited to use in radio receivers. They may be applied-with equalsuccess to other systems which are to operate in such a way as to beinitially unresponsive to ihput'voltages below a predetermined minimumvalue. Additional advanamplifier employing a vacuum tube havingascreen-grid, an audio-frequency amplified employing a vacuum tubehaving a control-grid,-a

loud speaker, and meansfonpreventing response.

from said loud speaker until a signal voltage having an initialamplitude exceeding ap're'detere mined valueis impressed upon saidmeans, said means including a rectifier connected to rectify said signalvoltage, a resistor across which there is developed a positive voltageof such value as to maintain said high-frequency amplifier at lowamplifying efiiciency, a connection by which said positive voltage isapplied to said screen-grid, a resistor across'which there is developeda negative voltage sufficient to maintain said audi-frequency amplifiercompletely inoperative, a connection for applying said negative voltageto said control-grid, and a time-delaying network for sequentiallychanging said positive and negative voltages, first on 'said screen-gridand then on said control-grid, as soon as said signal voltage exceedssaid predetermined value, to values which increase the amplifyingefiiciency of said highireque'ncy amplifier and render saidaudio-Irequency amplifier operative.

2. A radio receiver including high-frequency and audio-frequencyamplifiers; a loud speaker; and automatic regulating means for reducingthe amplification of said amplifiers to prevent re' spouse from saidloud speaker whenever the ampiitude of the signal impressed across saidmeans is less than a certain value, for maintaining said amplifiers insaid condition of reduced amplification until said amplitude exceeds asubstantially higher value, for sequentially restoring the amsaidamplifiersn. i

plification of 'said high-frequency'amplifier and said audio-frequencyamplifier as soon as said amplitude exceeds said higher value, and formaintaining said amplifiers in said condition oi restored amplificationuntil said signal falls below said first-mentioned value; saidautomatic- 3. A radio receiver including a high-frequency amplifyingvacuum tubehaving a screen-grid; an audio-frequency amplifying vacuumtube having a control-grid; a loudspeaker; and automatic regulatingmeans for preventing response fromsaid loudspeaker whenever theamplitude of the signal impressed across said means is less than acertain value and until said signal exceeds a substantially highervalue, andfor permitting response from said loud speaker as soon as saidsignal ex'ceeds said higher value; said automatic regulating meansincluding 'a' rectifier, a net- 'work associated with said rectifier andconnected to sources of positive and negative potential, connectionsfrom said network to said screengrid and said control-grid,and'tirneedelayi ng elements in said connections.

4. A radio receiver including ahigh-frequency amplifying vacuum tubehaving a control-grid,

a screen-grid, and an output circuit; a second vacuum tube arranged tofunction both .as a recti-' fler to produce a; direct-current voltageand as, an amplifier of said direct-current voltage, saidrectifier beingcoupled to the'output circuit of said high-frequency amplifying vacuumtube and said amplifier having output circuit includ: ing a resistor; athird vacuum tube arr'angedto function as an' audio-frequencyamplifierand having a control electro'de;"a direct-current con; nection fromsaidscreen-grid to the low-potene tial' terminal of said resistori a.network consisting of a second and a third resistor in series'co'nnectedbetween said low potential' terminal and,

a source of negative potential; and a time-delay- -ing connectionbetween the junction 01' said sec-- 0nd and third resistors and saidcontrol electrode. v

5. A radio receiver including in combination,

tuning means, automatic resonance adjusting means for bringing saidreceiver into substantially' exact resonance with a high-frequencyvsignal selected by approximate setting of said tuning means, and meansfor causing said auto matic resonance adjusting means to release saidsignal when said tuning means reachesa point displaced a predeterminedamount from the setting which would correspond to exact resonance withsaid signal; said resonance adjusting means including a director unitand a corrector unit,

tuning means, automatic resonance adjusting means for bringing saidreceiver into substantially exact resonance with a high-frequency signalselected by approximate settingv of said tuning means, means forpreventing response from said receiver until said signal has aninitialamplitude exceeding a predetermined minimum value, and means independentof said responsepreventing means for releasing said signal when saidtuning means reaches a point displaced a '30 predetermined amount fromthe setting which would correspond to exact resonance therewith,

including a potential-dividing and polarity-reversing network whichrenders said resonance producing means increasingly unstable as saidtuning means is moved away from said setting of exact resonance.

7. The method of preventing undesired sound reproduction in a radioreceiving system pro- .viding high-frequency and audio-frequencyamplifications and tunable to selectively receive any one of a number ofdifferent carriers throughout a wide range of frequencies, whichcomprises restraining said amplifications when the ampli-.

tude of the selected carrier is less than a'prev determined minimumvalue and until said amplitude exceeds'a value substantially highei thansaid minimum value, and removing saidirestraint of said high-frequencyamplification and of said audio-frequency amplification in the ordernamed as soon as said amplitude exceeds said higher value.

8. The method of preventing undesired sound reproduction in a radioreceiving system employing a vacuum tube having a screen-grid for highafrequency amplification and a vacuum. tube hav- 'ing a-control-grid foraudio-frequency amplification, and tunable to selectively receive anyone of a number of different carriers throughout a wide range offrequencies, which comprises applying amplification restrainingpotentials on said screen-grid and on said control-grid when theamplitude of the selected carrier is less than a. predetermined minimumvalue and until said amplitude exceeds a value substantially higher thansaid minimum value, and removing said screen-grid and from saidcontrol-grid in the order named as soon assaid amplitude. exceeds saidhigher value.

9. .A radio receiver including in combination, j tuning means for tuningsaid receiver over a -ra'nge of frequencies, resonance adjustingmeansifor automatically bringing said receiver into substantialiy exactresonance-with a first highwas frequency signal selected by approximatesetting or said tuning means but for releasing said signal when saidtuning means is adjusted to select a second signal, and means connectedto and actuated by said resonance adjusting means for causing immediaterelease of said first signal. a

10. A radio receiver including in combination, tuning means for tuningsaid receiver over a range of frequencies, resonance adjusting means\for automatically bringing said receiver into substantially exactresonance with a first high- Irequency signal selected by approximatesetting 0 said tuning means but for releasing said signal when saidtuning means is adjusted to select a second signal, and means foraccelerating the release of said firstsignal by said resonance adforautomatically bringing said receiver into justing means, saidireleaseeaccelerating means including a potential-dividing andpolarityreversing "network so arranged as to'render said resonanceadjusting means increasingly unstable as said tuning means is moved awayfrom the setting which corresponds to exact resonance with said firstsignal.

11. A radio receiver including in combination, tuning means for tuningsaid receiver over a range of frequencies, means for automaticallybringing said receiver into substantially exact resonance with a firsthigh-frequency signal selected by approximate setting of said tuningmeans, means for preventing response from said receiver unless saidfirst signal has an initial amplitude exceeding a predetermined minimumvalue, means'for causing said resonance \adjusting means to release saidfirst signal when said tuning means is adjusted to select a secondsignal, and means independent of said response preventing means 'for'accelerating the release of said first signal by said resonanceadjusting means.

12. A radio receiver including in combination, tuning means fortuningsaid receiver over a range '01 frequencies, resonance adjustingmeans for automatically bringing said receiver into substantially exactresonance with a high-trequency signal selected by setting said tuningmeans between limits above and below the setting which corresponds toexact resonance with said signal, and means connected to and actuated'bysaid resonance adjusting means for causing immediate release of saidsignal as said tuning means is adjusted from a point within said limitsto a point outside said. limits.

13. A radio receiver. including in combination, tuning means for tuningsaid receiver over a range of frequencies, resonance adjusting means forautomatically bringing said receiver into substantially exact resonancewith a first modulated high-.irequency signal selected by approximatesetting of said tuning means, means for preventing response from saidreceiver unless said first signal has an initial amplitude exceeding acertain minimum value, said resonance adjusting means being such thatmodulation peaks in said-first signal cause release of said firstsignalwhen said tuning means is adjusted, to select a second signal, andmeans independent or said response-preventing means for accelerating thereleaseot said first signal by said resonance adjusting means. p

l4.'A radio receiver including in combination, tuning means for tuningsaid receiver over a range of frequencies, resonancendiusting meanssubstantially exact resonance with a high-trequency signal selected bysetting said tuning means between limits above and below the settingwhich corresponds to exact resonance with said signal, means i'orpreventing responsetrom said receiver. unless said signal .has aninitial amplitude exceeding a certain minimum. value, and meansindependent of said response-preventing means for accelerating therelease of said signal by said resonance adjusting means when saidtuning means is adjusted from a point within said limits to a pointoutside said limits,

said limits being so chosen that said resonance adjusting mea'nsreleasessaid signal beiore'distortion of said signal due to inaccurate settingof said tuning means becomes appreciable.

15. A radioireceiverincluding in combination, tuning means for'tu'ningsaid receiver over a range of frequencies, resonance adjusting means forautomatically bringing said receiver into substantially exact resonancewith a first highfrequency signal selected by approximate setting ofsaid tuning means but for releasing said Lfirstfsignal when said tuningmeans is adjusted to select a second signal, means for automaticallyregulating the high-frequency amplificatiomoi.

said receiver inversely in accordance with the strength of said firstsignal, and means connected to and actuated by said resonance adlustingmeans for causing immediate release 0! said first signal.

16. A radio receiver including in combination, tuning means for tuningsaid receiver over a range of frequencies, resonance adjusting means forautomatically bringing said receiver into substantialiy exact resonancewith a first highfrequency signal selected by approximate setting ofsaid tuning means but for releasingsaid first signal when said tuningmeans is adjusted to select a second signal, means for automaticallyregulating the high-frequency amplification, 01 said receiver inverselyin accordance with the strength of said first signal, means forpreventing response from said receiver unless said first signal has aninitial amplitude exceeding a:certain minimum value, and meansindepe'ndentoi said response-preventing means for accelerating therelease of said first signal by said resonance ad-u justing means.

1'7. A radio receiver including in combination, tuning means for tuningsaid receiver overa range or frequencies, resonance adjusting means forautomatically bringing said receiver into substantially exact resonancewith a first highfrequency signal selected by approximate setting ofsaid-tuning means but for releasing said first signal when said tuningmeans is adjusted to select a second signal, means but for preventingresponse from said receiver whenever the amplitude of thesignalimpressed across said means is less than a certain value and until saidamplitude exceeds a substantially higher value, and means independent ofsaid response-pneventing means for accelerating the release 01 saidfirst signal by said resonance adjusting means.

18. A radio receiver including in combination, tuning means, resonanceadjusting means for automatically bringing said receiver intosubstantially exact resonance with a first highfrequency signal selectedby approximate setting of said tuning means but for releasing said firststrength of said first signal, means for preventing response from mid-receiver whenever. the amplitude of the signal impressed across saidmeans is less than a certain value and until said amplitude exceeds 'asubstantially higher value, and means independent of saidresponsepreventing means for accelerating the release of said firstsignal by said resonance producing means.

19. The method of preventing undesired sound reproduction in a radioreceiving system tunable to selectively receive any one of a number ofdiiferent carriers throughout a wide range of frequencies and providingresonance adjustment for automatically bringing said system intosubstantially exact resonance with a first carrier selected byapproximate tuning. thereof-hut releasing said first carrier when saidsystem is tuned to receive a second carrier, which comprises maintainingsaid system in a substantially inoperative condition whenever theamplitude of said first carrier is less than a certain minimum value anduntil said amplitude exceeds .a value substantially higher than saidminimum value, rendering said system operative as soon as said amplitudeexceeds said higher value, and independently accelerating the release ofsaid first carrier.

20. The method of preventing undesired sound reproduction in a radioreceiving system tunable to selectively receive any one of a number ofdifierent carriers'throughout a; wide range of frequencies and providingresonance adjustment for automatically bringing said system intosubstantially exact resonance with a first carrier selected byapproximate tuning thereof but releasing said" first carrier when saidsystem is tuned to receive a second carrier and also pro-.

viding automatic regulation of the high-frequency amplification of saidsystem inversely in accordance with the amplitude of said first carrier,

- which comprises maintaining said system in sub- 'stantiailyinoperative condition whenever the amplitude of said first carrier asmodified by said automatic high-frequency amplification regulatln-isless than a certain minimum value and until said modified amplitudeexceeds a value substantially higher than said minimum value. renderingsaid system operative as soon as said modified amplitude exceeds saidhigher value, and independently accelerating the release of said firstcarrier,

SIDNEY WHITE.

. CERTIFICATE OF CORRECTION. Patent No. 2,165, 596. July 11, 1959.

SIDNEY Y. WHITE. It is hereby certified that error appears in theprinted specification of the above numbered patent requiring correctionas follows: Page L1,, secent Office 0nd column, line 27, after the word"amplifying" insert a comma; page 5-, first column, line 51, for"'lawer" read lower; same page, second column, line 27, for "well" readwill; line 52,- for "direct" read director; page '2, second column, line69, claim 1, for theword "amplified" read amplifier; page 8, firstcolumn, line 7-8, claim 1, for audi-fre'truency" read audiofrequency;page 9, second column, line 57, claim 17, strike out the word Jbut"; andthat the said Letters Patent should be read with this correction thereinthat the same may conform to the record of the case in .the Pat- Signedand sealed this 22nd day of August, A. D. 1959.

Leslie Frazer (Seal) Acting Commissioner of Patents.

